Point smoke detectors that use scattered light, that is, photoelectric detectors, are known in the art. For example, known photoelectric detectors include a sensing chamber into which smoke enters, an optical system to generate a flash of light and detect the light scattered by smoke particulate, and an electronic system to process and transmit a signal proportional to the smoke. The optical system includes an emitter and a receiver that are located in such a way so that a high amount of the light projected by the emitter is collected by the receiver only in the presence of the smoke when the smoke particulate scatters the light.
In known photoelectric detectors, smoke detection based on optical scattering is affected by inaccuracy sources other than the smoke that cause the light to scatter. For example, known inaccuracy sources include, but are not limited to (1) dust accumulation on sensing chamber surfaces that cause a slow output signal drift and an increase in smoke sensitivity, (2) small objects or insects that settle on walls of the sensing chamber and that cause quick output signal variations and false alarms, (3) moisture and steam effects that cause unwanted deviations of light radiation, false alarms and signal faults, and (4) temperature variations that cause emitter and receiver characteristic changes.
Although a high amount of the light projected by the emitter is collected by the receiver only in the presence of the smoke, a small amount of the light projected by the emitter is also collected by the receiver even in the absence of the smoke. In this manner, a low level output signal or a clean air value is generated. It is known to use the clean air value to monitor the correct operating conditions of the emitter and the receiver and to easily and quickly calibrate a detector in a factory.
The clean air value of an output signal is typically generated by the light from the emitter being reflected multiple times within the sensing chamber. Accordingly, the clean air value typically has depended on the geometrical configuration, roughness, and color of the sensing chamber. However, when the light reflects multiple times within the sensing chamber, the clean air value of the output signal is more sensitive to the inaccuracy sources described above so unwanted signal variations, false alarms, and sensitivity variations occur more easily.
The above-identified disadvantages can be minimized when the sensing chamber is designed so that the clean air value is zero. However, such designs introduce more costs to calibrate the detectors.
Accordingly, some photoelectric detectors have been designed to include two optical chambers, and U.S. Pat. No. 3,968,379, U.S. Pat. No. 7,872,815, and U.S. Pat. No. 4,870,394 disclose examples of such detectors. However, each of these detectors includes a second receiver, for example, a reference receiver, in addition to a smoke receiver that is illuminated by part of emitter radiation in a compensating or reference chamber. These two receivers are connected in a bridge circuit and an electrical signal output from the bridge circuit is used to generate an alarm signal. Accordingly, the smoke receiver is not directly monitored. Furthermore, because two receivers are required, these types of detectors require more opto-electronic and electronic components and therefore, are more expensive to manufacture and implement.
The goal of any double chamber scheme is to reduce clean air signal drift due to dust and dirt accumulating during the detector's lifetime. However, it is preferred that any double chamber scheme includes an effective configuration that is low cost to manufacture and implement. This is particularly so in two cases: (1) when a smoke detector is in an environment where a standard photoelectric smoke detector gets dirty quickly and (2) when a smoke detector includes a high sensitivity or high gain photoelectric detector.
In view of the above, there is a continuing, ongoing need for a smoke detector with an improved double optical chamber.